Support is requested to continue experiments on the hormonal regulation of behavioral sexual differentiation in the ferret. Studies are proposed to determine (a) whether administering testosterone (T) to female ferrets on postnatal days 5-15, in concentrations which mimic the known plasma concentrations of T normally present in males during this neonatal period, will cause behavioral masculinization equivalent to that present in control males, (b) if particular sites in the female ferret forebrain can be identified into which T implanted neonatally enhances their ability to display masculine coital behaviors in adulthood, (c) whether neonatal administration of T, either s.c. or intracerebrally, influences the preference of female ferrets to approach male versus female sexual partners, (d) what the plasma concentrations of estradiol and estrone are in male and female ferrets during perinatal development and whether sex differences in the activity of aromatase in hypothalamus + preoptic area exist at the same perinatal ages, (e) whether the ability of male and female ferrets to display feminine sexual behavior in adulthood can be reduced by perinatal treatments which restrict the amount of estrogenic stimulation available to the developing brain.